Dry thermal interface material

ABSTRACT

A thermal interface material including a compound which has high thermal conductivity, is dry-to-the-touch, but naturally tacky, and may be formed into various shapes, such as sheets and blocks, to serve as a heat transfer material for electronic components. The compound includes a pre-blend of a polyol ester and an antioxidant, a boron nitride filler, a high viscosity oil, and either a solvent, a surfactant, and a polystyrene-based polymer, or aluminum silicate. A method for using the compound includes the steps of providing a heat generating electronic component with a first surface; providing a heat dissipating component with a second surface with which the first surface is to interface; and disposing the compound between the respective surfaces to effectuate efficient heat transfer therebetween. Further, a removable liner can be applied to an exposed surface of the compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of allowed U.S. application Ser. No.09/951,501, filed Sep. 14, 2001 now U.S. Pat. No. 6,610,635, andentitled DRY THERMAL INTERFACE MATERIAL, which is a continuation-in-partof U.S. application Ser. No. 09/661,729, filed Sep. 14, 2000, andentitled: “DRY THERMAL GREASE”, now U.S. Pat. No. 6,475,962.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to thermal transfer materials and, moreparticularly, to a dry thermal interface material, and related methodsof producing and applying the material between an electronic componentand a heat sink.

2. Description of the Related Art

Electronic assemblies are usually fabricated with a plurality ofelectronic components attached to a substrate, such as a printer circuitboard. In order for these assemblies to operate properly and reliablyfor extended periods of time, the heat generated by the components mustbe efficiently and reliably transferred from the component to the board,which acts as a heat sink.

Such electronic assemblies are operating at increasingly highertemperatures as they are built smaller and run faster. With smallerelectronic components, the density can also be increased, which furtherincreases the need for efficient and reliable removal of heat.

The ultimate theoretical thermal transfer occurs where a component andthe heat sink interface in continuous contact. In reality, however, therespective surfaces of the component and heat sink have irregularities,such as microscopic voids or pores, which trap air. Since air is a poorconductor of heat, these irregularities/voids must be filled with somethermally conductive material, to effect more efficient thermaltransfer. The following materials and techniques have been used topromote this thermal transfer.

Silicone-based thermal grease served as an early thermal interfacematerial for electronic assemblies. Such grease is formed by dispersingthermally conductive ceramic fillers in silicone to form a sticky paste.

When the grease is applied between a surface of the electronic componentand a surface of the heat sink, the grease fills the voids andeliminates the interstitial air. Any excess grease flows out at theedges of the component. The use of this grease allows for the thinnestpossible joint as both mating surfaces come into contact at their highpoints, resulting in a very low thermal resistance.

Although such grease has proved to be a very good thermal conductor,problems are associated with its use. It is messy, due to itsmoist-to-the-touch, sticky state, and it is time-consuming to apply(e.g., generally the right amount of grease must be applied). Also, ifthe grease is applied to a protective sheet liner, to facilitatehandling, shipping, etc., when the liner is removed prior to applicationof the grease to the electronic component surface, up to 50% of thegrease may remain on the liner, causing waste, increasing costs, andresulting in a less effective thermal interface than desired. Inaddition, during operation of the electronics, when heat is beinggenerated, the thermal grease migrates away from the area ofapplication. Also, silicon-based greases exhibit the disadvantage ofcausing silicone contamination of a wave solder bath. If silicone oilmigrates onto a printed circuit board, any solder re-work on the boardwill not adhere. Such migration may also cause short circuits on theboard.

Non-silicone thermal grease was then developed to address many of theabove-discussed problems associated with silicone-based products.Non-silicone greases are formed by dispersing the thermally conductiveceramic fillers in hydrocarbon oils.

While the non-silicone-based greases addressed themigration/contamination characteristics of silicone-based products, theystill suffered from being messy, since they still exhibited moist/stickycharacteristics, and they were still difficult and time-consuming toapply.

In a further effort to provide an acceptable replacement for thermalgrease, relatively thicker and drier elastomeric thermal pads weredeveloped. Their composition is basically silicone rubber-containingheat-conducting particles, such as zinc oxide, aluminum oxide, aluminumnitride, and boron nitride. The advantages of using these pads haveincluded the facts that they are less messy (due to being drier),installation is easier and less time-consuming, and they eliminate theneed to apply only the correct amount of grease with each application.

As noted above, however, the ultimate thermal interface is where twoparts touch at as many points as possible, and only where microscopicvoids appear, are they filled. Whereas the above-described grease flowseasily into these voids, and is easily displaced to allow as much directcontact as possible between the component and the heat sink, these padsdo not allow for any direct contact between the surfaces of thecomponent and the heat sink. That is, these silicone elastomers deformto surface irregularities only when a significant compressive load isapplied, which may be detrimental to the electronic component. At lowpressure, the pad simply cannot fill the air voids between the surfaces,causing a relatively very high thermal resistance.

Wax or paraffin-based phase-change materials have also been developed,which exhibit grease-like thermal performance and, due to their relativedryness, exhibit easier elastomeric pad-like handling and installation.These phase-change materials have been used in a stand-alone form, havebeen reinforced with fiberglass, or have been coated onto foil orKapton®. Kapton is a thermally conductive but electrically insulativepolymide film available from the DuPont company. These phase-changematerials are solid at room temperature, but they behave much likethermal pastes or greases once they reach their phase-change, or meltoperational temperature, i.e., usually between 40° C. and 70° C.

Since these phase-change materials are solid and dry at roomtemperature, they are not messy to apply. As they are heated they becomeliquid and flow into the pores. However, in a vertical orientation ofthe electronic component, they will flow out of the interface, againleaving voids. These materials require pressure sensitive adhesives toadhere to parts during assembly, which adhesives undesirably increasethermal resistance. The operational high temperature range forphase-change materials is only 150° C., however, versus 200° C. forthermal grease. Further, in a “cold plate” application, i.e., usingwater and/or thermoelectric modules to help cool electronic assemblies,the temperature would not reach the melt operational temperature, so thephase-change material would not receive enough heat to melt into place(wet the surface), and therefore would not be useable, whereas greaseworks at such temperature. Further, each thermal cycle and subsequentphase-change may introduce new air voids that may not be refilled.

In light of the above, thermal pads are easy to use, but exhibit arelatively high thermal resistance. And, while phase-change materialsmay outperform pads in terms of thermal transfer efficiency, they stillbear limitations in use and performance. Thermal grease offers superiorperformance to these grease replacements, including most particularlythe lowest thermal resistance, but can be very messy and labor-intensiveduring application.

Although the prior art described above eliminates some of the problemsinherent in the thermal transfer art, this prior art still does notdisclose or teach the most efficient compound and related methods ofproduction and use.

SUMMARY OF THE INVENTION

Accordingly, it is a purpose of the present invention to provide athermal interface material that exhibits the positive attributes ofconventional thermal greases, but it easier to apply.

It is another purpose of the present invention to provide a dry thermalinterface material that allows for total wetting action to fill anyvoids between an electronic component and a heat sink, without the needto change the phase of the material.

It is another purpose of the present invention to provide a thermalinterface material having a positive coefficient of thermal expansionand thixotropic properties to improve wetting action, therebyfacilitating total thermal interface contact between an electroniccomponent and a heat sink.

It is yet another purpose of the present invention to provide a thermaltransfer material that allows immediate heat transfer at any operationaltemperature, without the need for a phase-change, making the materialparticularly appropriate for cold plate applications.

It is further a purpose of the present invention to provide a thermaltransfer material that offers the advantages and conveniences of thermalpads and phase-change materials, but has the superior performance ofthermal grease.

It is still another purpose of the present invention to provide athermal grease which facilitates handling and prevents migration.

It is but another purpose of the present invention to provide anon-silicone- and nonwaxed-based thermal grease or paste that may benaturally tacky.

It is another purpose of the present invention to provide a thermaltransfer compound which can be molded into sheets, blocks and otherforms, and then cut, to facilitate placement between an electricalcomponent and a heat sink.

It is also a purpose of the present invention to provide a drop-in-placethermal transfer compound that is easy to use and handle in manymanufacturing environments.

It is also a purpose of the present invention to provide a thermaltransfer compound which can be applied with minimal pressure.

It is still another purpose of the present invention to provide a drybut naturally tacky thermal transfer material that does not require anyadhesive or other additives which might reduce thermal transferefficiency.

It is also a purpose of the present invention to provide a thermaltransfer compound that is thixotropic in nature to prevent run out frombetween an electronic component and a heat sink during operation of thecomponent.

It is another purpose of the present invention to provide a method forforming a relatively dry thermal transfer compound, but with sometackiness.

It is also a purpose of the present invention to provide a thermaltransfer material that exhibits both superior thermal transference andelectrical insulative properties.

It is another purpose of the present invention to provide a method formore effectively applying a thermal transfer material between anelectronic component and a heat sink.

It is also a purpose of the present invention to provide a method forapplying a thermal transfer material using only a minimal amount offorce to effect total interface contact between an electronic componentand a heat sink.

To achieve the foregoing and other purposes of the present inventionthere is provided a thermal transfer material including a compound thathas high thermal conductivity, is relatively dry-to-the-touch, isnaturally tacky, and may be formed into various shapes, such as blocks,sheets, etc., to facilitate its application between an electroniccomponent and a heat sink. The compound includes a pre-blend made up ofa polyol ester, and an antioxidant, as well as a filler(s), a highviscosity oil, and either a polystyrene-based polymer, a solvent, and asurfactant, or aluminum silicate.

The present invention is also directed to providing a method forproducing the compound including the steps of mixing polyol ester in anamount of about 99 wt. percent and an antioxidant in an amount of about1 wt. percent to form the pre-blend (which pre-blend makes up about 8-12wt. percent of the compound), adding at least one of a zinc oxide fillerin the amount of about 18-80 wt. percent and a magnesium oxide filler inthe amount of about 60 wt. percent to the pre-blend; and adding a highviscosity oil in the amount of about 2.5-5.5 wt. percent. Further, inone embodiment there is added a surfactant in the amount of about 0.2wt. percent, a polystyrene-based polymer in the amount of about 3 wt.percent, and a solvent in the amount of about 1 wt. percent. In analternate embodiment, instead of the polymer, solvent and surfactant,there is added aluminum silicate in an amount of about 5.2 wt. percentof the compound.

This dry-to-the-touch thermal transfer material offers very low thermalresistance at lower closure pressure, like conventional thermal grease,but offers the handling ease of the conventional grease replacementsdiscussed above, thereby eliminating the need to sacrifice thermalperformance for convenience. The block, sheet, etc. forms of thematerial can be die-cut and possess a natural tackiness that allows themto adhere to an electronic component or heat sink without usingadditional adhesives that would degrade thermal performance. Thematerial also exhibits a positive coefficient of thermal expansion andexhibits thixotropic properties which allow it to wet surfaces, furtherimproving interface contact. And, because heat transfer beginsimmediately and can take place at any temperature, it is excellent forcold plate applications. The material is also silicone free to avoidproblems of silicone contamination, and can be electrically insulating,if desired.

The present invention is also directed to a method for providing athermal interface material for electronic component assemblies,including the following steps: providing a heat generating electroniccomponent with a first mounting surface; providing a second mountingsurface on a heat sink upon which the first mounting surface of the heatgenerating electronic component is to be mounted; and disposing the drythermal transfer material discussed above between the first mountingsurface and the second mounting surface to effect heat transfer from theelectronic component to the heat sink.

Further, the material can include a thermally conductive foil backing,or a thermally conductive and electrically insulative backing, and bedie cut, if desired. Also, removable liners can be applied to exposedsurfaces of the compound to facilitate handling, shipping and storage,but same are removed prior to the material being applied between theelectronic component and the heat sink.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a non-silicone, non-wax-based, dry-to-the-touchthermal grease or paste compound that is naturally tacky, and can beused as a very efficient thermal transfer material, such as between anelectronic component and a heat sink.

This compound is based on a pre-blend made up of about 98.8% by weightof polyol ester, such as HATCOL 2373, and about 1.2% by weight of anantioxidant, such as ETHANOX 330. To this pre-blend are added at least afiller(s), an oil, a polymer, a surfactant and a solvent, or afiller(s), the oil, and aluminum silicate, as described below, dependingupon the intended use for the composition.

According to a first embodiment, exemplary components and weightpercentages are set forth in the following Table 1:

TABLE 1 Components Percentage by Weight of Compound Pre-Blend 8.1 to 9.9Zinc Oxide 65.52 to 80.08 High Viscosity Oil 4.5 to 5.5 Polymer 2.7 to3.3 Solvent  .9 to 1.1 Surfactant .198 to .202

The zinc oxide is a powder that serves as the filler material. Asfillers, any thermally conductive filler known in the art should besuitable, including other metal oxides, silver, aluminum nitride andboron nitride. Magnesium oxide would generally not be used as a fillerwith this embodiment because of its relatively large particle size.

The oil is preferably polyisobutene, which is generally known as aviscosity index improver. One commercially available oil is calledIndopol.

This embodiment of the compound is made thixotropic and relativelydry-to-the touch by incorporating an appropriate polymer. The polymer ispreferably polystyrene-based. The thixotropic characteristic providescomplete wetting across the interface, and keeps the material fromflowing out of the interface, even in a vertically-oriented application.The dryness significantly facilitates handling and use.

Exemplary solvents are naphtha, hexane, heptane, and otherquick-dissipating petroleum distillates.

An exemplary surfactant is polyglycolether, sold commercially asGenapol. The surfactant facilitates the formation of the grease compoundinto a thin film. The surfactant is not needed, however, in thecomposition for the second “block” embodiment described below, which ismore dense.

This compound performs as well as, or better than conventional thermalgrease, as shown in Table 2 below, which compares the typical propertiesof conventional thermal grease with the dry-to-the-touch compound of thepresent invention:

TABLE 2 Invention Typical Properties Standard Grease Compound TestMethod Consistency/Penetration 320 190 ASTM-D217 Evaporation loss 200°C., %/wt 0.5 0.1 FTM-321 Thermal Conductivity, W/m ° K 0.70 1.0 ASTMD-5470 modified Thermal Resistance, ° C. in²/W 0.05 0.03 ASTM D-5470modified Volume Resistivity, ohm-cm 1.65 × 1014 2.0 × 101″ ASTM D-257

The compound according to the present invention is, like phase-changeand elastomeric pad materials, dry-to-the touch, but does not otherwisesuffer from the above-discussed drawbacks of these materials. On theother hand, the compound according to the present invention does notexhibit the messiness of conventional thermal grease, but enjoys itsvery low thermal resistance.

The compound exhibits a positive coefficient of expansion, resulting inswelling of the compound to wet the surface, helping to fill the voidseven better than conventional grease. The positive coefficient ofexpansion occurs on the microscopic level, so there is no physicalchange in the compound.

The compound can be applied directly to an electronic component or to asubstrate, prior to the electronic component being applied to thesubstrate.

This compound can also be combined with, e.g., a propellant and appliedby spraying directly onto the electronic component or substrate in thedesired thickness, as would be known in the art. The compound of thisfirst embodiment can also be screen printed directly onto the electroniccomponent or substrate. In either case, the compound will becomedry-to-the-touch within a few seconds of application due to theevaporation of the solvent.

Alternatively, the compound can be layered on a first protective liner.Then a second protective liner can be placed over the still exposedsurface of the compound, opposite the first protective liner to form alaminate. These liners should be made of a material that has a highrelease value that doesn't disturb the compound film. A preferredmaterial is polypropylene. The liners simply function to protect thefilm until the film is applied. The laminate can be cut to sizedepending upon the size of the base of the electronic component.

In order to apply the compound, the first liner is removed from thelaminate. The surface of the compound that is thereby exposed is thenplaced against the base surface of the electronic component with“rolling finger pressure”. Then the second liner is removed from thecompound which remains adhered to the base of the electronic component.That is, the compound will adhere, due to its natural tackiness, to thecomponent or heat sink without adhesive or any other “non-thermallyconductive” material that would degrade thermal performance. Theelectronic component is then placed on the substrate with pressure ofabout 5 PSI or more to achieve total interface contact.

As can be seen, application of the thermal transfer compound of thepresent application is possible with only minimal pressure. In this way,there is less likelihood of damage to the sensitive electroniccomponents.

Alternatively, after the first liner is removed, and the compound isadhered to the component or heat sink, the second, exposed liner canremain on to protect the material, i.e., the components can bepre-coated for easy and protected transportation, storage and laterassembly.

The material can be cold applied and requires no heating or curingthereafter. The material can be removed easily and cleanly, ifnecessary, without special tools for easy access and rework.

Alternatively, the compound may be applied as a thin layer on a heatconductive sheet backing to produce a thermal transfer material that canbe cut to fit a variety of different shapes of the electricalcomponents.

For example, the compound of the first embodiment of the presentinvention can be coated onto an aluminum foil substrate referred toherein as “A” to provide exceptional thermal conductivity, or coatedonto Kapton®, “K”, for both electrical insulation and thermalconductivity, as described in detail below. Among the advantages areagain its ease of use (as with thermal pads and phase-change materials),as well as the superior performance of thermal grease.

In the former regard, the dry compound is pre-applied to both sides of afoil substrate A in a thickness varying preferably from 1 mil to 4 mil.For example, a 2 mil foil could have 1 or 2 mil of film applied to eachopposing surface, as shown in Table 3 below. Of course, athinner/thicker substrate, and/or a thinner/thicker layer of thecompound can be applied thereto, as desired.

TABLE 3 Substrate Type Aluminum(Ex.1) Aluminum(Ex.2) Physical PropertiesSubstrate Thickness, in. 0.002 0.002 Compound Thickness (per side),0.001 0.002 in. Total Thickness, in. 0.004 0.006 Thermal PropertiesThermal Resistance, ° C. in²/W 0.018 0.02 (ASTM D-5470 modified)

This embodiment is an easy-to-use, non-paraffinic thermal interface filmwith the superior performance of conventional thermal grease. Unlikephase-change materials, this film can be used at 25° C. or lower, makingthe compound well-suited for cold plate applications. Further, thecompound requires only minimal force to achieve total surface wetting,and interface contact. As seen, the film exhibits an exceptionally lowthermal resistance; as low as about 0.02 (C° in²/W).

The foil A with the compound thereon can be cut into any desired sizeand shape to adapt to the interface surface of the electronic component,and be attached thereto. These cut thermal interface materials can alsobe applied to a roll of sheet-like material for shipping/storing, priorto application on the electronic component.

Similarly, the compound can be applied to the substrate K, whichexhibits not only thermal transfer capabilities, but also electricinsulative properties, as illustrated in Table 4:

TABLE 4 Value Test Method Physical Properties Substrate type Kapton ®Substrate Thickness, in. 0.002 Compound Thickness/(per side) in. 0.002Total Thickness, in. 0.006 Thermal & Electrical Properties ThermalResistance, ° C. in²/W 0.028-0.03 ASTM D-5470 (modified) DielectricStrength, V/mil (VAC) 2000 (12000) ASTM D-149 Dielectric Constant, @ 1KHz 3.7 ASTM D-150 Volume Resistivity, ohm-cm 1.01 × 10¹⁵ ASTM D-257

This thermal interface material is thus a die-cut polymide, electricallyinsulating substrate K coated on both sides with the dry thermalinterface compound. This material offers high heat transfer and highelectrical insulating capabilities, and has a high cut-throughresistance, as shown in Table 5, which compares the typical propertiesof the thermal interface materials A and K according to the presentinvention:

TABLE 5 A K Test Method Physical Properties Substrate Aluminum Kapton ®Substrate Thickness, in. 0.002 0.002 Compound Thickness/Side, in. 0.0020.002 Total Thickness, in. 0.006 0.006 Thermal & Electrical PropertiesThermal Conductivity, W/m ° K 2.5 0.77 ASTM D-5470 modified ThermalResistance, ° C. in²/W 0.02 0.028-0.03 ASTM D-5470 modified DielectricStrength, V/mil VAC N/A 2000 (12,000) ASTM D-149 Volume Resistivity,ohm-cm N/A 1.01 × 10¹⁵ ASTM D-257

As can be seen from the following Table 6, the present invention ineither the foil version A or the Kapton version K exhibits exceptionalthermal resistance at relatively low pressure, when compared with theconventional interface materials discussed above (using ASTMD-5470-modified):

TABLE 6 THERMAL RESISTANCE Kapton Fiberglass Aluminum Foil SiliconeReinforced Reinforced Reinforced Graphite A K Pad Phase Change Pad PhaseChange Pad Graphite Pad Pad 10 psi 0.023 0.029 0.094 0.054 0.057 0.030.029 30 psi 0.019 0.027 0.068 0.047 0.055 0.02 0.02 50 psi 0.005 0.0260.059 0.037 0.054 0.011 0.017 70 psi 0.002 0.024 0.052 0.034 0.05 0.0070.016

Further, like the foil embodiment A described above, protective linerscan be used, the entire thermal interface material combination can beapplied between an electronic component and the heat sink with apressure of about 5 PSI or more, and sheets of this embodiment K can beplaced on rolls for shipment and storage, prior to application.

Compared to the use of the compound-only applied directly to thecomponent/heat sink, as described above, the thermal conductivity of thefoil A or Kapton K embodiments is slightly less, due to the additionaluse of the foil and Kapton sheets.

In addition to the foil or Kapton substrates noted above, othersubstrates or carriers can be used, e.g., a fiberglass mesh.

According to still another embodiment of the compound, blocks, sheets,and other shapes can be molded to fill larger gaps between an electroniccomponent and a heat sink, much like the conventional elastomeric padsdescribed above. That is, as surface textures of an electronic componentand a heat sink and/or distances therebetween can be uneven, theseblocks can be shaped to fit any desired gap or shape between a componentand heat sink. Preferably the shape would be a flat, smooth, rectangularor circular sheet, etc., as known in the art. As with the embodimentdescribed above, this embodiment is dry-to-the-touch.

For this embodiment of the compound, referred to herein as the blockembodiment, the exemplary components and weight percentages are setforth in the following Table 7.

TABLE 7 Components P rcentages by Weight of Compound Pre-Blend 10.08 to12.32 Zinc Oxide Powder 18.09 to 22.11 Magnesium Oxide Powder 54.81 to66.99 Aluminum Silicate 4.68 to 5.72 High Viscosity Oil 2.25 to 2.75

The pre-blend and the oil are as described above.

The zinc oxide and the magnesium oxide are powders that serve as fillermaterials. Again, other known fillers can be used, as described above.

The aluminum silicate is a claylike material used to thicken thecompound relative to the above-described embodiment, so that it can beformed into these shapes. Thus, the chemistry for the second embodimentis generally similar to that of the first embodiment, except that thesecond is dryer, and more clay like due to the addition of the aluminumsilicate.

With this block embodiment, the compound can be formed with a thicknessgenerally much greater than those associated with the above-describedembodiments A and K. For example, the blocks could be 80 to 200 milsthick.

This block embodiment is highly conformable and naturally tacky, whichprovides an excellent replacement for the less conformable, conventionalsilicone elastomer gap fillers which require significant pressure toachieve 100 percent surface contact. As described above, such highpressure could damage the electronic component. The block embodiment ofthe present invention fills the gap and displaces the air with much lesspressure being exerted on the electronic component.

The block embodiment's highly conformable nature allows the pad to fillall voids between a heat generating device and heat sink. Thenon-silicone formula thereof is particularly advantageous for opticalapplications and high compression loads. The material also conducts heataway from individual components and into metal covers, frames orspreader plates. This embodiment also offers unique advantages inapplications such as microprocessors, cache chips, heat pipe interposerplates, laptop PCs, high-density handheld portable electronics,electronic ballasts and various automotive applications.

The typical properties of the block embodiment are set out in thefollowing Table 8:

TABLE 8 Physical Properties Value Test Method Composition Non-SiliconeColor Gray Density 2.8 ASTM D-70 Thickness, in. (mm) 0.08 (2 mm) & upOperating Temperature Range −40° C. to 150° C. Thermal Conductivity, W/m° K 1.68 ASTM D-5470 (modified) Thermal Resistance, ° C. in²/w/mil 0.03Coefficient of Thermal Expansion 31.8 × 10⁻⁶/° C. Dielectric Strength,V/mil 318 ASTM D-149 Volume Resistivity, ohm-cm 2.15 × 10¹⁵ ASTM D-257

Unlike with the foil/Kapton embodiment described above, there is nobacking attached to the block embodiment. Nevertheless, polypropyleneliners can again be used, if desired.

The blocks, sheets, etc., that are formed can be die-cut to exactspecifications from about 0.08″ (2 mm) and higher.

This block embodiment will conform to any shape and/or size of acomponent enabling complete physical contact so as to minimize theresistance to heat flow and to achieve the best thermally conductivepath.

Unlike phase-change materials, not only does this embodiment requireonly minimum pressure, heat transfer starts at 25° C., and total wettingaction can again be attained via a positive coefficient of thermalexpansion, without the need to change phase.

In comparison with the relatively thin interface material embodiments Aand K described above, and the relatively thick blocks or sheetsembodiment described above, the present invention can, if desired, beformed in thicknesses therebetween. For example, thicker substrates orcarriers, such as a fiberglass mesh, as well as thicker layers of thecompound of the present invention can be used.

Further, in contrast to the use of an electrically insulative materiallike Kapton, it may be desirable to have not only thermal conductivitybut also electrical conductivity. An example of such a structure wouldbe a copper foil with a silver layer thereon.

Specific suggested applications for the thermal interface materialaccording to the above-described embodiments of the present inventioninclude: power modules, IGBTs, DC-DC converter modules, solid staterelays, diodes, power MOSFETs, RF components and thermoelectric modules;microprocessors, multichip modules, ASICs and other digital components;power amplifiers, large area applications for power supplies and othercustom enclosure heat dissipating surfaces.

As can also be seen from the above description, the present inventionexhibits at least the following advantages over the prior art: a)retains all of the proven values of conventional thermal grease; b)requires only minimum force for total interface contact between anelectrical component and a heat sink; c) allows for total “wettingaction” to fill voids between an electrical component and a heat sinkwithout changing phase; d) exhibits a positive coefficient of thermalexpansion and thixotropic properties to increase the wetting action foreven greater interface contact; e) allows heat transfer immediately andtherefore can be used at any operational temperature, unlikeconventional phase-change materials, making the material an excellentselection for cold plate applications; f) provides essentially a“drop-in place” product that is easy to use and handle in anymanufacturing environment; g) provides a non-messy, dry-to-the-touch,naturally tacky material using no separate adhesive, or othernon-conductive material (e.g., fiberglass) that may affect thermalresistance; h) microscopically changes to fill voids on electronic partsurfaces; i) prevents run-out due to its thixotropic nature; j) canexhibit both heat transfer properties and high electrical insulatingcapabilities; and k) will not lose its viscosity and become runny afterapplication, under continuous heat or pressure conditions, or if appliedto a vertical surface.

The foregoing is considered illustrative only of the principles of theinvention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed. Accordingly, all suitable modifications and equivalents maybe resorted to that fall within the scope of the invention and theappended claims.

1. A thermal transfer compound, comprising: a polyol ester, anantioxidant, a boron nitride filler, a high viscosity oil, a surfactant,a polystyrene-based polymer and a solvent.
 2. The compound as recited inclaim 1, wherein the polyol ester and the antioxidant form a pre-blendwith the polyol ester making up about 99 wt. percent of the pre-blend,and the antioxidant making up about 1 wt. percent of the pre-blend. 3.The compound as recited in claim 2, wherein the pre-blend is in theamount of about 8 to 12 wt. percent of the compound.
 4. A thermaltransfer compound, comprising: a polyol ester, an antioxidant, a boronnitride filler, a high viscosity oil and aluminum silicate.
 5. Thecompound as recited in claim 4, wherein the polyol ester and theantioxidant form a pre-blend with the polyol ester making up about 99wt. percent of the pre-blend, and the antioxidant making up about 1 wt.percent of the pre-blend.
 6. The compound as recited in claim 5, whereinthe pre-blend is in the amount of about 10 to 12 wt. percent of thecompound.
 7. A thermal interface material, comprising: a thermallyconductive compound made of a polyol ester, an antioxidant, a boronnitride filler, a high viscosity oil, a surfactant, a polystyrene-basedpolymer and a solvent, and a sheet receiving, on at least one surfacethereof, the compound.
 8. The material as recited in claim 7, whereinthe sheet is polypropylene.
 9. A method for providing a thermalinterface material for electronic component assemblies, comprising thefollowing steps: a) providing a heat generating electronic componentwith a first surface; b) providing a substrate with a second surfacewith which the first surface is to interface; c) disposing the compoundproduced according to claim 1 on a first surface of a sheet; d) placingthe first surface of the sheet on the first surface of the heatgenerating electronic component with the compound therebetween; e)removing the sheet; and f) placing the second surface of the substrateon the first surface of the heat generating electronic component withthe compound therebetween to effectuate heat transfer between thecomponent and the substrate.
 10. The method as recited in claim 9,wherein the compound is disposed by the following steps: g) placing arelatively thin layer of the compound onto the first surface of thesheet; h) cutting the sheet into a shape corresponding to a shape of thefirst surface of the heat generating electronic component; and i)placing the cut sheet on the first surface of the heat generatingelectronic component with the compound therebetween.
 11. A method forproviding a thermal interface material for electronic componentassemblies, comprising the following steps: a) providing a heatgenerating electronic component with a first surface; b) providing asubstrate with a second surface with which the first surface is tointerface; c) disposing the compound produced according to claim 4 on afirst surface of a sheet; d) placing the first surface of the sheet onthe first surface of the heat generating electronic component with thecompound therebetween; e) removing the sheet; and f) placing the secondsurface of the substrate on the first surface of the heat generatingelectronic component with the compound therebetween to effectuate heattransfer between the component and the substrate.
 12. The method asrecited in claim 11, wherein the compound is disposed by the followingsteps: g) forming a block of the compound to fit a shape of a gapbetween the first surface of the component and the second surface of thesubstrate; h) placing a surface of the block on a sheet; i) placing theblock and sheet in the gap between the first surface of the componentand the second surface; j) removing the sheet; and k) applying pressureto at least one of the component and substrate.
 13. A non-water solublethermal transfer compound, comprising: a polyol ester, an antioxidant, aboron nitride filler, a high viscosity oil, a surfactant, apolystyrene-based polymer and a solvent.
 14. A non-water soluble thermaltransfer compound, comprising: a polyol ester, an antioxidant, a boronnitride filler, a high viscosity oil and aluminum silicate.
 15. Anon-water soluble thermal interface material, comprising: a thermallyconductive compound made of a polyol ester, an antioxidant, a boronnitride filler, a high viscosity oil, a surfactant, a polystyrene-basedpolymer and a solvent, and a sheet receiving, on at least one surfacethereof, the compound.
 16. A method for forming a thermal interfacematerial, comprising the steps of: forming a thermally conductivecompound from a polyol ester, an antioxidant, a boron nitride filler, ahigh viscosity oil, a surfactant, a polystyrene-based polymer and asolvent; coating a sheet, on at least one surface thereof, with thecompound; and removing the sheet.
 17. A method for forming a thermalinterface material, comprising the steps of: forming a thermallyconductive compound from a polyol ester, an antioxidant, a boron nitridefiller, a high viscosity oil, a surfactant, a polystyrene-based polymerand a solvent; coating a sheet, on a surface thereof, with the compoundto form a film; allowing at least part of the solvent to evaporate;placing the sheet on a heat generating device with the compoundtherebetween; removing the sheet; and placing a heat sink on thecompound.
 18. A method for providing a thermal interface material on aheat generating component, comprising the following steps: a) providingthe heat generating component with a first surface; b) providing asecond surface on a heat dissipating component upon which the firstsurface of the heat generating component is to be mounted; and c)disposing a thermal interface material including a polyol ester andboron nitride between the first and second surfaces to effectuate heattransfer from the heat generating component to the heat dissipatingcomponent.
 19. The method of claim 18, whereby the disposing step is bythe following steps: d) shaping a block of the material to fit a shapeof a gap between the first and second surfaces; and e) placing the blockin the gap between the first and second surfaces.
 20. The method ofclaim 18, whereby the disposing step is by the following steps: d)disposing a relatively thin layer of the material onto a sheet; e)attaching the material and sheet to said one of the first and secondsurfaces; and f) removing the sheet.